When drumming with my regular band we often only have a PA which the vocals go through and the guitars and bass use their own amps. I sometimes cannot hear the vocals at all. Would this product help with this situation as well if I can get the vocals from the desk sent to the headphones so I can at least hear them in one ear whilst still picking up the instruments in the free ear.

Can you also please let me know what leads, cables I would need in order to connect to a mixing desk. Also if I would need to purchase something to stop unexpected noise into the headphones and damaging my ears/hearing.

I am by no means a master at electronic stuff but I have dabbled a bit. But it seems to me that a simple analog mixer would serve the purpose and would provide a bit more versatility. Your laptop can be plugged in from its 1/8" audio jack into the RCA tape inputs with a simple cable. Whatever type of synth you are using is most likely either and XLR jack or 1/4" cable which would easily hook to one of the main channels and then you have your headphone out with its own level adjustment. This gives you a bit more control over the individual levels rather than a simple headphone amp. A sound guy can easily tie into the main output for the P.A. system.